The present invention relates to improvements in methods and apparatus for the vacuum deposition of a coating of material onto aluminum webbing, and in particular to the deposition of such a coating onto a moving web as the web moves from an unwinding coil to a winding coil. The invention is particularly suited, and will be described with reference to, the deposition of a high reflective surface onto an aluminum web.
The basic method for the vacuum deposition of a high reflective surface onto an aluminum web has been practiced for many years and is thus well known in the art. This well known method has been practiced, however, only on webbing in the form of individual sheets of glass or aluminum, typically 40 inches wide, 0.020 inches thick and 50 inches long, weighing approximately four pounds. In the present invention, a coil of aluminum typically 40 inches wide, 0.020 inches thick and from 1000 to 1500 feet long, weighing 900 to 1400 pounds or more is unwound from an unwinding coil onto a winding coil and the high reflective coating is vacuum deposited onto the traveling web in stages as the web travels between the unwinding coil to the winding coil.
The method of the present invention, like that of the prior art, is effective to transform a web of polished anodized aluminum (called lighting sheet) having a reflectivity of about 85% into lighting sheet having a reflectivity of about 95%. This increase in reflectivity can be significant when, for example, the sheet is used for reflectors for increasing the light output from lighting fixtures.
The basic, known method includes the following steps. A sheet of aluminum is inserted into a vacuum chamber which is subdivided into series of internal vacuum compartments which are separated by seals that permit each compartments to be at the particular pressure suitable for the process being performed within that compartment. The aluminum sheet is transported from one compartment to another to permit whatever process is being performed in that compartment to be applied to the sheet.
In the first compartment the aluminum sheet is exposed to heat to drive out water. In the next compartment the sheet is exposed to a glow discharge, typically maintained by argon gas, to drive out any remaining water and for further heating to facilitate adhesion. Next the sheet is placed into a compartment where a thin oxide layer is applied to increase adhesion and seal the anodized (aluminum dioxide) layer which covers the polished aluminum sheet stock.
In the next compartment, an opaque layer of aluminum is deposited onto the web. In the following compartments the classical reflectance-enhancing layers of a quarter wave length of low index of refraction material then a quarter wave length of a high index of refraction material are deposited. The high index of refraction material is of the kind deposited in an atmosphere of a partial pressure of oxygen and a partial pressure of water vapor. Each coating is applied in a vacuum compartment to which the sheet is transported after the preceding coating has been applied.
The present invention improves the method and apparatus for applying a high reflectance coating to an aluminum web by using a continuously moving web from a coil of aluminum, by using the water vapor created by heating the web to fuel the glow discharge, and by using the water vapor created by heating the web to stabilize the pressure and background atmosphere in the compartment where the high index layer is applied.